Integrated gasification and electrolysis process

ABSTRACT

Aspects of the invention relate to improvements in the flexibility with which oxygen and hydrogen, for example from electrolysis, may be supplied to processes having both gasification and methanation steps, as well as improvements in how such processes may be operated in response to variations in carbonaceous feeds. Offsets, between the ideal quantity of hydrogen and the quantity available from a given source may be compensated for by adjusting one or more operations of the process, and in particular such operation(s) that ultimately impact the quantity of CO and/or CO2 available downstream of the gasifier for conversion to methane in an RNG product stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional application No.62/697,540, filed Jul. 13, 2018, which is incorporated by reference inits entirety.

FIELD OF THE INVENTION

Aspects of the invention relate to gasification processes for theproduction of renewable natural gas (RNG), with such processes beingintegrated with electrolysis for supplying oxygen and hydrogen feeds.

DESCRIPTION OF RELATED ART

The gasification of coal has been performed industrially for over acentury in the production of synthesis gas that can be further processedinto transportation fuels. More recent objectives of developing energyindependence with reduced greenhouse gas emissions has led to a stronginterest in using biomass as a gasification feed, and thereby apotential source of renewable natural gas (RNG). According to amulti-step process, biomass gasification is performed first, by partialoxidation in the presence of an oxidant gas containing oxygen and otherpossible components such as steam. Gasification at elevated temperatureand pressure, optionally in the presence of a catalytic material,produces an effluent with methane and oxides of carbon (CO, CO₂), aswell as hydrogen. This effluent, which may be referred to as syngas inview of its CO and H₂ content, is normally treated to remove a number ofundesired components such as particulates, alkali metals, sulfurcompounds, and tars (generally referring to organic compounds having aboiling point temperature greater than that of benzene). Such treatmentsteps render the gasifier effluent suitable for downstream conversion ofthe high concentrations of H₂ and CO/CO₂ by catalytic methanation, inorder to increase the methane content in a resulting RNG stream.

As may be appreciated from the atomic composition of wood and otherforms of biomass, the gasifier effluent generally lacks sufficienthydrogen for complete conversion of the feed carbon content to methane,resulting in an excess of CO and CO₂ that must be considered in theoverall process economics. The addition of hydrogen to allow morecomplete utilization of biomass carbon in producing methane is thereforeof considerable interest. Hydrogen is often available as a productobtained from steam reforming of methane and/or other hydrocarbons,according to a technology that is very robust but suffers from thesignificant amount of energy required. In addition, the use of fossilmethane to produce such hydrogen defeats the purpose of obtainingnatural gas from purely renewable sources. The integration of hydrogenfrom electrolysis, as a potential source to allow more completemethanation of gasifier effluents, has been proposed by Gassner et al.(ENERGY 33 (2008): 189-198) and Dace et al. (ENERGY PROCEDIA 75 (2015):801-806). Further developments in biomass gasification, to improveeconomics relative to fossil-derived natural gas production, arenonetheless needed to realize its commercial scale implementation in theproduction of RNG.

SUMMARY OF THE INVENTION

Aspects of the invention are associated with the discovery ofgasification processes for the production of methane, having improvedefficiencies in the management of feeds input to, and/or energy derivedfrom, such processes. Providing electrolysis oxygen and hydrogen to suchprocesses can advantageously reduce the overall carbon footprintassociated with the production of RNG from a carbonaceous feed such ascoal or biomass. For example, electrolysis oxygen can be used to augmentthe quantity supplied to a gasifier from a conventional air separationunit (ASU), whereas the co-generated electrolysis hydrogen can beprovided to a methanation reactor and thereby improve the utilization ofcarbon in the feed for methane production. This, in turn, createsfurther opportunities for integration, particularly in terms of theadditional heat generation from the exothermic methanation reaction thatcan be recovered in the form of steam and ultimately electricity topower the electrolyzer.

Particular aspects relate to improvements in the flexibility with whichelectrolyzer oxygen and hydrogen may be supplied to processes havingboth gasification and methanation steps, as well as improvements in howsuch processes may be operated in response to variations in carbonaceousfeeds. In this regard, a typical ASU is associated with high capital andoperating costs, and its capacity may often be misaligned with the needfor pure oxygen in the gasifier. This can occur, for example, if an ASUis unavailable in a proper size to be economical and/or supplementalsources of oxygen such as pressure swing adsorption (PSA) cannot providesufficient purity. Both ASU and PSA are recognized for their potentialto generate oxygen, but they also produce nitrogen that is not alwaysreadily utilized or monetized. Whereas hydrogen may be available fromconventional reforming, this production method is energy intensive andis normally based on fossil-derived hydrocarbons. Electrolysis thereforehas the potential to address a number of shortcomings associated withthe production of purified oxygen and/or hydrogen by other methods, inaddition to offering improvements in overall feed carbon utilization(e.g., methane yield) and heat integration.

The quantity of electrolysis oxygen, whether or not supplied incombination with other sources such as ASU oxygen or PSA oxygen, may bedetermined primarily or entirely by the quantity and type ofcarbonaceous feed to the gasifier. However, the corresponding amount ofelectrolysis hydrogen produced, in satisfying this oxygen demand, isgenerally not optimal for complete conversion of CO and CO₂, generatedby gasification, in the downstream methanation reactor. Advantageously,such offsets, between the ideal quantity of hydrogen and the quantityavailable as the stoichiometric amount obtained from water electrolysisto satisfy the oxygen demand of the gasifier, may be compensated for byadjusting one or more operations of the process, and in particular suchoperation(s) that ultimately impact the quantity of CO and/or CO₂available downstream of the gasifier for conversion to methane in an RNGproduct stream. Such operation to be adjusted may alternatively, oradditionally, impact the CO/H₂ molar ratio and/or the CO₂/H₂ molar ratiodownstream of the gasifier. Otherwise, if sufficient electrolysishydrogen is available, representative processes comprising gasificationand methanation may exclude such operation(s) altogether, therebysimplifying RNG production and reducing the associated costs.

Embodiments of the invention are directed to processes for producingmethane, for example as an RNG product, with such processes comprisingboth gasification and methanation. Gasification refers to the contactingof a carbonaceous feed, in a gasifier (or, more particularly, agasification reactor of a gasifier), with an oxygen-containing gasifierfeed, to provide a gasifier effluent comprising CO, CO₂, H₂, and/or H₂O,and generally all of these components. At a point upstream of amethanation reactor, or otherwise within the methanation reactor,electrolysis hydrogen obtained from an electrolyzer may be combined withthe gasifier effluent. The added, or makeup, electrolysis hydrogen, orat least a portion thereof, is reacted in the methanation reactor withat least a portion of the CO and/or CO₂ in the gasifier effluent to formmethane and thereby improve the utilization of carbon in thecarbonaceous feed, in the production of RNG. Advantageously, anoperation of the process, for example one or more of a series ofoperations performed on the gasifier effluent downstream of the gasifierand upstream of the methanation reactor, may be adjusted in response tothe makeup quantity or flow rate of the electrolysis hydrogen, toimprove overall process efficiency. The operation that is adjusted isnamely one that affects the concentration of CO and/or CO₂ in the feedto the methanation reactor, with representative operations being a sourshift operation or an acid gas removal operation. The operation that isadjusted may alternatively, or additionally, be one that affects theCO/H₂ molar ratio and/or the CO₂/H₂ molar ratio of the feed to themethanation reactor.

Other embodiments of the invention are directed to integratedgasification, methanation, and electrolysis processes for producing RNG.Representative processes comprise contacting, in a gasifier of agasification zone, biomass with an oxygen-containing gasifier feed toprovide a gasification zone effluent comprising CO, CO₂, H₂, and/or H₂O,and generally all of these components. The oxygen-containing gasifierfeed comprises electrolysis oxygen obtained from electrolysis of waterin an electrolyzer. The processes further comprise converting, in amethanation reactor of a methanation zone, at least a portion of the COand/or the CO₂ in the gasification zone effluent to methane. The RNG isobtained as, or the RNG is recovered from (e.g., following one or moreoptional purification steps), a methanation zone effluent. Thegasification zone or the methanation zone, or both, comprise at leastone operation affecting the CO concentration or the CO₂ concentration,or causing a change in such concentration from the feed to thatoperation to the product of that operation. Alternatively, or incombination, the gasification zone or the methanation zone, or both,comprise at least one operation affecting the CO/H₂ molar ratio and/orthe CO₂/H₂ molar ratio, or causing a change in such molar ratio from thefeed to that operation to the product of that operation. The processesmay yet further comprise adjusting the at least one operation inresponse to a makeup quantity or flow rate of electrolysis hydrogen.This makeup quantity may be the entire flow rate of electrolysishydrogen added to the process in one or more locations between thegasifier and the methanation reactor (e.g., added to either thegasification zone or the methanation zone, or both), with some or all ofthis makeup quantity being reacted to form methane in the methanationreactor. This makeup quantity of electrolysis hydrogen may be asubstantially stoichiometric amount, or substantially twice the molarflow rate of electrolysis oxygen added to the gasifier in theoxygen-containing gasifier feed, which may comprise one or more separategaseous feed streams.

The gasification zone and/or methanation zone may comprise one or moreoperations as described herein, which may or may not affect, or may atleast substantially affect, the methanation reactor feed or inlet COconcentration or CO₂ concentration. Alternatively, or in combination,the gasification zone and/or methanation zone may comprise one or moreoperations as described herein, which may or may not affect, or may atleast substantially affect, the methanation reactor feed or inlet CO/H₂molar ratio and/or CO₂/H₂ molar ratio. The gasification zone may beseparated from the methanation zone by a compression stage.

Yet other embodiments of the invention are directed to processes forproducing methane, with such processes comprising gasifying acarbonaceous feed to provide a gasifier effluent comprising CO, CO₂, H₂,and/or H₂O, and generally all of these components. The processes alsocomprise introducing a makeup quantity of hydrogen to the methanationreactor or upstream of the methanation reactor, to react at least aportion of the makeup hydrogen with at least a portion of the CO and/orCO₂ in the gasifier effluent to form methane. Such processes may furthercomprise adjusting an operation of the process in response to the makeupquantity of hydrogen. Advantageously, representative processes mayexclude at least one processing step that would otherwise affect themethanation reactor feed, or methanation reactor inlet, CO concentrationor CO₂ concentration. Alternatively, or in combination, such processesmay exclude at least one processing step that would otherwise affect themethanation reactor feed, or methanation reactor inlet, CO/H₂ molarratio and/or CO₂/H₂ molar ratio. This impact on concentration or molarratio may be caused by consuming or producing CO and/or CO₂, by reactionin the operation (and therefore in the overall process), for example inthe case of a sour shift operation. The excluded processing step(s) maybe that/those which would otherwise affect the methanation reactor feed,or methanation reactor inlet, CO concentration or CO₂ concentration, byadding CO and/or CO₂ to, or removing CO and/or CO₂ from, the operation(and therefore the overall process), for example in the case of an acidgas removal operation. The excluded processing step(s) may be that/thosewhich would otherwise alternatively affect, or which would otherwiseadditionally affect, the methanation reactor feed, or methanationreactor inlet, CO/H₂ molar ratio and/or CO₂/H₂ molar ratio, by suchaddition and/or removal of CO and/or CO₂, to and/or from the operation(and therefore the overall process).

These and other embodiments, aspects, and advantages relating to thepresent invention are apparent from the following Detailed Description.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the exemplary embodiments of thepresent invention and the advantages thereof may be acquired byreferring to the following description in consideration of theaccompanying figures, in which the same reference numbers are used toidentify the same features.

FIG. 1 depicts a flowscheme illustrating generally the integration of anelectrolyzer with a gasification zone and a methanation zone to producemethane according to representative processes described herein.

FIG. 2 depicts a flowscheme illustrating particular operations withinthe gasification zone and methanation zone that may be carried out inprocess illustrated in FIG. 1.

The same numbers are used between FIGS. 1 and 2 to designate the same orsimilar elements. In order to facilitate explanation and understanding,these figures provide a simplified overview. Associated equipment suchas vessels, heat exchangers, valves, instrumentation, and utilities, isnot shown, as its specific description is not essential to theimplementation or understanding of the various aspects of the invention.Such equipment would be readily apparent to those skilled in the art,having knowledge of the present disclosure. Other processes forproducing methane according to other embodiments within the scope of theinvention, having configurations and constituents determined, in part,according to particular processing objectives, would likewise beapparent.

DETAILED DESCRIPTION

The expressions “wt-%” and “mol-%,” are used herein to designate weightpercentages and molar percentages, respectively. The expressions“wt-ppm” and “mol-ppm” designate weight and molar parts per million,respectively. For ideal gases, “mol-%” and “mol-ppm” are equal topercentages by volume and parts per million by volume, respectively.

Embodiments of the invention are directed to a process for producingmethane from a carbonaceous feed by gasification and methanation,performed in a gasifier and methanation reactor, respectively, in whicha source of makeup hydrogen is added to the process to improve theoverall utilization of carbon in the carbonaceous feed in producingmethane (i.e., to improve methane yield, based on the feed carboncontent). If the source of makeup hydrogen is obtained from theelectrolysis of water, then advantageously this reactant, as opposed tohydrogen generated from the steam reforming of fossil hydrocarbons, is acarbon-free resource that does not contribute to the carbon footprintassociated with the methane that is produced. According to someembodiments, therefore, the methane may be produced predominantly, ifnot entirely, from renewable feeds, including biomass as thecarbonaceous feed, air and/or electrolysis oxygen, and electrolysishydrogen. Moreover, some or all of the electrical energy needed for theelectrolysis may be obtained as heat recovered from the gasifier orexothermic methanation reactor.

In a gasifier (or, more particularly, a gasification reactor of agasifier), the carbonaceous feed is subjected to partial oxidation inthe presence of an oxygen-containing gasifier feed in an amount thatthis generally limited to supply only 20-70% of the oxygen that would benecessary for complete combustion. The oxygen-containing gasifier feedmay also comprise other oxygenated gaseous components including H₂Oand/or CO₂. A gasifier effluent is produced, comprising CO, CO₂, andmethane (CH₄) that are derived from the carbon present in thecarbonaceous feed, as well as H₂ and/or H₂O, and generally both,together with other components in minor concentrations. The carbonaceousfeed may comprise coal (e.g., high quality anthracite or bituminouscoal, or lesser quality subbituminous, lignite, or peat), petroleumcoke, asphaltene, and/or liquid petroleum residue, or otherfossil-derived substance. In a preferred embodiment, the carbonaceousfeed may comprise biomass. The term “biomass” refers to renewable(non-fossil-derived) substances derived from organisms living above theearth's surface or within the earth's oceans, rivers, and/or lakes.Representative biomass can include any plant material, or mixture ofplant materials, such as a hardwood (e.g., whitewood), a softwood, ahardwood or softwood bark, lignin, algae, and/or lemna (sea weeds).Energy crops, or otherwise agricultural residues (e.g., loggingresidues) or other types of plant wastes or plant-derived wastes, mayalso be used as plant materials. Specific exemplary plant materialsinclude corn fiber, corn stover, and sugar cane bagasse, in addition to“on-purpose” energy crops such as switchgrass, miscanthus, and algae.Short rotation forestry products, such as energy crops, include alder,ash, southern beech, birch, eucalyptus, poplar, willow, paper mulberry,Australian Blackwood, sycamore, and varieties of paulownia elongate.Other examples of suitable biomass include vegetable oils, carbohydrates(e.g., sugars), organic waste materials, such as waste paper,construction, demolition wastes, digester sludge, and biosludge.Representative carbonaceous feeds therefore include, or comprise, any ofthese types of biomass. Particular carbonaceous feeds comprising biomassinclude municipal solid waste (MSW) or products derived from MSW, suchas refuse derived fuel (RDF). Carbonaceous feeds may comprise acombination of fossil-derived and renewable substances, including thosedescribed above.

Downstream of the gasifier, a methane product may be obtained from aneffluent of the methanation reactor directly, or otherwise recoveredfrom this effluent following additional purification and/or treatment.Carbon utilization to produce methane, and consequently methane yield,in the overall process are improved by virtue of adding the makeuphydrogen (e.g., improved relative to a baseline process in which themakeup hydrogen is absent). For example, carbon utilization and/ormethane yield (e.g., based on carbon present in biomass or othercarbonaceous feed) may be increased by at least about 3%, at least about5%, or at least about 10%, relative to a baseline process in which nomakeup hydrogen (e.g., electrolysis hydrogen) is added. For comparativepurposes, such baseline process may have all gasifier, or gasificationzone, parameters maintained the same, while the methanation reactor, ormethanation zone, is operated to maximize methane yield according to thebaseline process.

Importantly, processes as described herein may be operated to addressconstraints associated with the use of electrolysis hydrogen and oxygen,in which the electrolysis oxygen requirement may be governed all orpredominantly by the amount and type of carbonaceous feed to begasified, whereas the corresponding amount of electrolysis hydrogengenerated may be sub-optimal for converting all CO and CO₂ obtained fromthe gasifier to methane. In this case, one or more operations of theprocess, which affect the CO and/or CO₂ concentrations of themethanation reactor inlet, may be adjusted or controlled (e.g., bycontinuous feedback) to reduce waste and enhance overall processeconomics. The one or more operations to be adjusted may alternativelyor additionally affect the CO/H₂ and/or CO₂/H₂ molar ratios of themethanation reactor inlet.

In some embodiments, this adjustment or control can advantageously beused to maintain complete or substantially complete conversion of COand/or CO₂ in the methanation reactor (or methanation zone), such that ahigh quality methane product may be obtained directly or recovered, forexample, with properties understood in the art to render it of “pipelinequality.” A representative methane product, or RNG product in the caseof being derived from a renewable carbonaceous feed such as biomass, mayhave any one, or any combination, of (i) a methane concentration of atleast about 90 mol-% (e.g., from about 90 mol-% to about 99 mol-%), orat least about 95 mol-% (e.g., from about 95 mol-% to about 98 mol-%),(ii) a concentration of hydrocarbons other than methane (e.g., a totalconcentration of C₂-C₆ hydrocarbons) of less than about 5 mol-%, or lessthan about 3 mol-%, (iii) a hydrogen concentration of less than about0.05 mol-%, or less than about 0.01 mol-%, (iv) a CO₂ concentration ofless than about 2 mol-%, or less than about 1 mol-%, and/or (v) a sulfurconcentration of less than about 10 mol-ppm, or less than about 5mol-ppm. Other properties of the methane product, or RNG product,rendering it suitable for pipeline transport include having a heatingvalue within 5% of that of pure methane and being free of water andtoxic or corrosive contaminants. In some embodiments, this product mayhave a hydrogen concentration of up to about 4 mol-% (e.g., from about 1mol-% to about 4 mol-%) and nonetheless be considered pipeline quality.Generally, the CO concentration of this product is less than about 1mol-%, such as less than about 0.1 mol-%.

Representative operations, which may be adjusted by virtue of theirimpact on the amount(s) and/or concentration(s) of CO and/or CO₂ at theinlet of the methanation reactor (e.g., in the total methanation reactorfeed or gas entering this reactor), or otherwise by virtue of theirimpact on the CO/H₂ and/or CO₂/H₂ molar ratios at the inlet of themethanation reactor, include operations that (i) produce or consume COand/or CO₂ in the process (e.g., generate or produce CO and/or CO₂ inthe process by reaction, or otherwise react CO and/or CO₂ and therebydeplete or consume one or both of these components from the process), or(ii) add and/or remove CO or CO₂ in the process (e.g., introduce anadditional source of CO and/or CO₂, or feed comprising CO and/or CO₂, tothe process or otherwise separate a stream comprising one or both ofthese components from the process). Such operations may cause a change(an increase or a decrease) in the absolute concentration, oralternatively the relative amount, of CO and/or CO₂ from the inlet ofthe operation to the outlet of the operation (excluding any flowbypassing the operation) of generally at least about 2% (e.g., fromabout 2% to about 30%), typically at least about 5% (e.g., from about 5%to about 35%), and often at least about 10% (e.g., from about 10% toabout 30%). More particularly, such operation(s) may be adjusted inresponse to a makeup quantity or flow rate of hydrogen (e.g.,electrolysis hydrogen), at least a portion of which is, in turn, reactedin the methanation reactor.

Therefore, in the case of an operation that produces or otherwise addsCO and/or CO₂, that operation may be enhanced, or increased, atrelatively high quantities of makeup hydrogen and diminished, ordecreased, at relatively low quantities of makeup hydrogen. Conversely,in the case of an operation that consumes or otherwise removes CO and/orCO₂, that operation may be diminished, or decreased, at relatively highquantities of makeup hydrogen and enhanced, or increased, at relativelylow quantities of makeup hydrogen. The ability to regulate, i.e.,enhance or diminish, one or more operations that affect (i) theconcentration, or overall amount, of CO and/or CO₂ at the methanationreactor inlet and/or (ii) the CO/H₂ and/or CO₂/H₂ molar ratio at themethanation reactor inlet, constitutes an important aspect of thepresent invention, in terms of improving carbon utilization of biomassor other carbonaceous feed.

An enhancement, or increase, in an operation may involve increasing itscapacity, for example by increasing the flow of a solvent or extractionagent for contacting with the feed to that operation, or otherwiseincreasing a quantity of solid bed material such as an adsorbent orcatalyst available for such contacting (e.g., by commissioning anadditional vessel containing such material). An enhancement mayalternatively involve increasing the severity of the operation, forexample by increasing operating temperature and/or pressure, and/orotherwise increasing residence time (e.g., decreasing space velocity).Conversely, a diminishment, or decrease, in an operation may involvedecreasing its capacity, for example by decreasing the flow of a solventor extraction agent for contacting with the feed to that operation, orotherwise decreasing a quantity of solid bed material such as anadsorbent or catalyst available for such contacting (e.g., bydecommissioning a vessel containing such material). A diminishment mayalternatively involve decreasing the severity of the operation, forexample by decreasing operating temperature and/or pressure, orotherwise decreasing residence time (e.g., increasing space velocity). Agiven operation may also be enhanced or diminished according to anadjustment in which all, or at least a portion, of the feed to theoperation is diverted from, for example bypassed around, the operation.In the case of bypass, the bypassed feed to the operation then simplybecomes part of a feed to a downstream operation, such as the nextsuccessive operation normally performed in the process. In the case of acomplete bypass of all of the feed to a given operation, the processessentially operates without that operation, or excludes that operation.

In the case of an operation that produces or otherwise adds CO and/orCO₂, an enhancement or increase would directionally produce or add moreCO and/or CO₂. In the case of an operation that consumes or otherwiseremoves CO and/or CO₂, an enhancement or increase would directionallyconsume or remove more CO and/or CO₂. Conversely, in the case of anoperation that produces or otherwise adds CO and/or CO₂, a diminishmentor decrease would directionally produce or add less CO and/or CO₂. Inthe case of an operation that consumes or otherwise removes CO and/orCO₂, a diminishment or decrease would directionally consume or removeless CO and/or CO₂.

Having knowledge of the present disclosure, those skilled in the artwill readily appreciate the full scope of adjustments that can beperformed to a given operation to enhance or diminish its performance,as needed, on the basis of the quantity of hydrogen (e.g., electrolysishydrogen) being added to the process.

Particular aspects of the invention also relate to process control,whereby one or more operations of the process that affect theconcentration, or overall amount, of CO and/or CO₂ at the methanationreactor inlet, and/or that affect the CO/H₂ and/or CO₂/H₂ molar ratio atthe methanation reactor inlet, may be adjusted in response to changes inthe makeup quantity or flow rate of hydrogen. For example, the controlmay be such that a given composition of the methanation reactor inletshould be maintained as a set point, with that composition allowing fora high utilization of CO and/or CO₂ in the conversion to methane. A setpoint composition, may, for example, be based on the concentration of H₂in the methanation reactor feed or inlet, relative to the concentrationsof CO and/or CO₂. A particular set point may be the concentration ratio(or molar ratio) according to [H₂]/(3.[CO]), or according to[H₂]/(3.[CO]+4.[CO₂]), in which the brackets are used to designatecomponent concentrations. The control may be such that this ratio,and/or some other measure relating to the efficiency with which COand/or CO₂ are utilized, is maintained at a value of 1 or nearly 1, forexample generally within the range of about 0.7 to about 1.5, typicallywithin the range of about 0.8 to about 1.3, and often within the rangefrom about 0.9 to about 1.1. In response to a deviation from this ratio,or other measure, from its set point, an operation affecting theconcentration, or overall amount, of CO and/or CO₂ at the methanationreactor inlet as described herein may be automatically or manuallyadjusted.

For example, in response to a deviation from the set point that isindicative of an excess concentration of hydrogen (e.g., due to anincreased amount or flow rate of makeup hydrogen), an operation thatproduces or otherwise adds CO and/or CO₂, may be enhanced or increased,or an operation that consumes or otherwise removes CO and/or CO₂, may bediminished or decreased. Conversely, in response to a deviation from theset point that is indicative of a deficient concentration of hydrogen(e.g., due to a decreased amount or flow rate of makeup hydrogen), anoperation that produces or otherwise adds CO and/or CO₂, may bediminished or decreased, or an operation that consumes or otherwiseremoves CO and/or CO₂, may be enhanced or increased. Adjustments toenhance or diminish an operation may be according to any of thoseparticularly described herein, with the extent of such enhancement ordiminishment possibly being controlled automatically or manually (e.g.,the extent of diverting a feed from, for example by bypassing the feedaround, an operation may be automatically or manually adjusted, inresponse to deviations from a set point relating to the efficiency withwhich CO and/or CO₂ are utilized in the methanation reactor.)

A particular operation affecting the methanation reactor inlet CO and/orCO₂ concentration is a sour shift operation, which refers to anoperation that performs a catalytic water-gas shift (WGS) reaction inthe presence of sulfur compounds. Therefore, a sulfur-tolerant WGScatalyst is used in a WGS reactor of this operation. The sour shiftoperation is used to produce hydrogen by the WGS reaction, according to:CO+H₂O→CO₂+H₂. Therefore, the sour shift operation, or any WGS operationthat may be used in the process generally, downstream of the gasifierand upstream of the methanation reactor, is an operation that consumesCO and produces both CO₂ and H₂. For purposes of characterizing a sourshift operation, or WGS operation in general, according to the types ofoperations described above, this may be considered “an operation thatconsumes or otherwise removes CO and/or CO₂,” as this operation, ifadjusted, would normally be diminished or decreased in response toexcess hydrogen and enhanced or increased in response to deficienthydrogen, due to the co-production of H₂ by the WGS reaction. In thecase of a reverse WGS operation, whether performed in the presence orabsence of sulfur compounds, this may conversely be considered “anoperation that produces or otherwise adds CO and/or CO₂.” Anotherparticular operation affecting the methanation reactor inlet CO and/orCO₂ concentration is an acid gas removal operation that is “an operationthat consumes or otherwise removes CO and/or CO₂,” since CO₂ is removedin this operation, whereas the amount of CO from the feed to the productis generally unaffected. Yet another particular operation that may beconsidered “an operation that produces or otherwise adds CO and/or CO₂”is the gasifier, as this operation, if adjusted, would normally beenhanced or increased in response to excess hydrogen and diminished ordecreased in response to deficient hydrogen. Whereas this operationco-produces H₂ as a result of gasification reactions, this component isgenerally produced at less than the stoichiometric ratio with respect toCO, according to the methanation reaction. In a particular example ofadjusting the gasifier operation, such that this operation isdiminished, or decreased, at relatively low quantities of makeuphydrogen (e.g., in response to a deviation from a set point that isindicative of a deficient concentration of hydrogen) a feed may bediverted from the gasifier. Such feed may be the oxygen-containinggasifier feed, comprising all or a portion of the oxygen (e.g.,contained in a makeup quantity of oxygen) that is fed to the process,with such oxygen comprising electrolysis oxygen and/or oxygen separatedfrom air.

With respect to any of the specific WGS (e.g., sour shift), reverse WGS,acid gas removal, or gasifier operations, or generally any otheroperation of the process that affects (i) the concentration, or overallamount, of CO and/or CO₂ at the methanation reactor inlet, and/or (ii)the CO/H₂ and/or CO₂/H₂ molar ratio at the methanation reactor inlet,such operation may be conveniently adjusted by diverting a feed from,for example by bypassing at least a portion of the feed around, thatoperation (e.g., using manual or automatic control) as described above.In the case of a sufficient makeup quantity or flow rate of hydrogen(e.g., electrolysis hydrogen), it may be expedient diminish anyoperation that consumes or otherwise removes CO and/or CO₂, to the pointat which such operation is not used at all. For example, the sour shiftoperation and/or the acid gas removal may be bypassed completely in thecase of achieving a desired utilization of carbon in the carbonaceousfeed. Accordingly, embodiments of the invention are directed toprocesses as described herein for the gasification of a carbonaceousfeed (e.g., biomass), followed by methanation, in which a source ofmakeup hydrogen is added to the process and the process excludes one orboth of a sour shift operation or an acid gas removal operation, suchthat, for example, the process involves, upstream of the methanationreactor (i) no substantial adjustment of the amount of CO and/or CO₂ inthe gasifier effluent, and/or (ii) no substantial adjustment of theCO/H₂ and/or CO₂/H₂ molar ratio in the gasifier effluent. That is, therelative amount of CO and/or CO₂ in the gasifier effluent may be changed(e.g., the number of moles of either of these components may beincreased or decreased) by less than about 10%, less than about 5%, oreven less than about 2%, by virtue of the absence of any operation thatsubstantially produces, adds, consumes, and/or removes CO and/or CO₂,and more particularly in the absence of any operation that substantiallyconsumes and/or removes CO and/or CO₂. Alternatively, or additionally,the relative CO/H₂ molar ratio and/or the relative CO₂/H₂ molar ratiomay be changed by these percentage ranges. In this manner,advantageously, substantially all CO and/or CO₂ produced in the gasifiercan be fed to, and preferably utilized in, the methanation reactor forthe production of the methane product (e.g., RNG).

Representative operations that may be adjusted in response to a makeupquantity of hydrogen can exclude those operations primarily used toremove components other than CO and/or CO₂, but which may nonethelessresult in some minimal change in the amount and/or concentration of COand/or CO₂ from the inlet of (or the feed input to) the operation to theoutlet of (or the product removed from) the operation. Such operationsthat may be excluded, i.e., not adjusted as described herein, encompassthose used for the removal of tar, solid particles, and sulfur, withparticular operations being a tar conversion operation, a gasfiltration/scrubbing operation, a supplemental cleaning operation, and asulfur removal operation, as discussed in greater detail below. Any ofthese operations, whether considered to reside in a gasification zone ora downstream methanation zone, may be performed on an effluent of thegasifier, prior to its introduction to a methanation reactor.

FIG. 1 illustrates generally the integration of an electrolyzer 35 witha gasification zone 100 and a methanation zone 200, for the productionof methane. This methane is namely present in a methanation reactoreffluent 25 from methanation zone 200, and is produced from carbon in acarbonaceous feed 10 that is fed to a gasifier of gasification zone 100.Also fed to this zone is a makeup quantity of oxygen 14 that may beobtained entirely as electrolysis oxygen 19 or may otherwise be providedas a combination of electrolysis oxygen 19 and optional air separationunit (ASU) oxygen 32, obtained from the separation of makeup air 40 inASU 45. ASU 45 can include any technology for separating pure orsubstantially pure oxygen, such as to obtain, for example, ASU oxygen 32and/or makeup quantity of oxygen 14 having an oxygen concentration of atleast about 90 mol-%, and preferably at least about 95 mol-%. ASU maytherefore include, or comprise, pressure swing adsorption (PSA) orvacuum pressure swing adsorption (VPSA), membrane separation, and/ormolten salt-based oxygen separation. The use of ASU 45 for removinginert nitrogen from makeup air 40 may be beneficial in terms of reducingequipment sizes, including the gasifier and equipment associated withdownstream processing operations, as well as improving the heating valueof the gasifier effluent and downstream products, with exemplarydownstream operations and products as described herein.

Makeup quantity of oxygen 14 may be determined at least partly (e.g.,predominantly or even entirely) on the basis of a makeup quantity ofcarbonaceous feed 10. According to some embodiments, makeup quantity ofoxygen 14 may be determined on the basis of both the makeup quantity andtype of carbonaceous feed, particularly if significant changes incharacteristics of carbonaceous feed 10 (e.g., its carbon content) occurover the course of a given operation. For example, the makeup quantityof oxygen may be the amount needed for a given degree of oxidation ofcarbon present in carbonaceous feed, such as a percentage of thestoichiometric oxygen requirement for oxidation to CO₂. Inrepresentative embodiments, the makeup quantity of oxygen, for example,may be generally from about 20% to about 95%, typically from about 30%to about 90%, and often from about 40% to about 80%, of thisstoichiometric requirement.

As is more clearly illustrated in FIG. 2, all or a portion of makeupquantity of oxygen 14 may be fed to gasifier 105 of gasification zone100, as oxygen-containing gasifier feed 14 a, with an optional portion14 b of this makeup quantity being used in tar conversion operation 110.Oxygen-containing gasifier feed 14 a may therefore comprise electrolysisoxygen 19 obtained from electrolyzer 35, and may optionally furthercomprise ASU oxygen 32 obtained from ASU 45. Oxygen-containing gasifierfeed 14 a can refer to all gases being fed or added to gasifier 105,whether or not combined upstream of, or within, gasifier 105. Forexample, oxygen-containing gasifier feed 14 a may further comprise otheroxygenated gaseous components such as H₂O and/or CO₂ that may becombined with electrolysis oxygen 19 and/or ASU oxygen 32 upstream ofgasifier 105, or otherwise introduced as a separate H₂O— and/orCO₂-containing feed 12 to gasification zone 100, or, more particularly,gasifier 105.

As is further illustrated in the more general flowscheme of FIG. 1, inaddition to electrolysis oxygen 19, electrolyzer also produceselectrolysis hydrogen 18, for example in a makeup quantity that is astoichiometric amount relative to electrolysis oxygen (e.g., 2 moles ofelectrolysis hydrogen per mole of electrolysis oxygen), from theelectrolysis of added water 30. Electrolysis hydrogen 18 may be combinedwith at least a portion of gasification zone effluent 16 to providecombined methanation zone feed 20. Alternatively, electrolysis hydrogen18 may be combined within methanation zone 200, for example with feedsto any of, or any combination of, operations of methanation zone 200,such as those operations shown in the more detailed flowscheme of FIG.2. According to a preferred embodiment, electrolysis hydrogen 18 iscombined with methanation reactor feed 212, or otherwise added directlyto methanation reactor 225. Regardless of where electrolysis hydrogen 18is added to the process (e.g., to the gasification zone 100 or to a feedto any of its constituent operations 105, 110, 115, 120, and/or 125, orto the methanation zone or to a feed to any of its constituentoperations 210, 215, 220, and/or 225), electrolysis hydrogen 18 canimprove the yield of methane, obtained in methanation reactor effluent25, by reaction with carbon oxides (CO and/or CO₂) that are generatedfrom the gasification of carbonaceous feed 10.

According to FIG. 1, further process integration may be realized by therecovery of gasification zone energy 15 from gasification zone 100and/or methanation zone energy 17 from methanation zone 200. This energymay be provided to electrolyzer 35, in the form of electrical energy, tomeet some or all of the requirements of electrolyzer 35. With referenceto FIG. 2, in view of the high temperatures used in gasifier 105, inaddition to the highly exothermic reactions occurring in methanationreactor 225, heat may be recovered from products downstream of theseoperations 105, 225. For example, gasifier heat recovery operation 115(e.g., comprising a heat exchanger) may be used to recover gasifier heat118, such as in the form of high pressure steam that is passed togenerator 121 that generates gasification zone energy 15 in the form ofelectricity. Likewise, methanation reactor heat recovery operation 230(e.g., comprising a heat exchanger) may be used to recover methanationreactor heat 218, such as in the form of high pressure steam that ispassed to generator 221 that generates methanation zone energy 17 in theform of electricity. In the embodiment illustrated in FIG. 2, gasifierheat recovery operation 115 recovers heat from hot tar conversionproduct 106, whereas methanation reactor heat recovery operation 230recovers heat from hot methanation product 216. Heat, and often lowgrade heat, may be recovered from the process, for example usinggasifier heat recovery operation 115 and/or methanation reactor heatrecovery operation 230, for the purpose of pre-heating and/orpre-treating water 30 that is used for electrolysis. Those skilled inthe art, having knowledge of the present disclosure, will appreciatethat various forms of heat may be recovered and utilized from any of anumber of streams of integrated processes described herein. Yet furtherpossibilities for process integration may arise from the recovery ofwater, for example water that is fed to and/or generated in gasifier105, from the process. Such water may be more specifically recovered(e.g., condensed) from the effluent of any operation described herein,including from the hot methanation product 216 and/or from themethanation reactor effluent 25 (e.g., as the result of methanationreactor heat recovery operation 230). Recovered water may advantageouslyprovide all or a portion of water 30 that is used for electrolysis, forexample in a manner such that external water is not required in theprocess (i.e., the process is self-sustaining, with respect to waterrecovery being sufficient to satisfy the water consumption requirement).

FIG. 2 provides a more detailed illustration of the process asillustrated in FIG. 1, but showing operations performed downstream ofthe gasifier 105 and upstream of the methanation reactor 225, which arenamely sequential operations that can be considered as being performedon the gasifier effluent 103, to provide methanation reactor feed 212.Each individual operation can be considered as associated with itsrespective feed and product streams, in which at least one property(e.g., composition, temperature, and/or pressure) of the product of theoperation is changed, relative to the feed to that operation. Also, aproduct of one operation can be considered as a feed to the next,subsequent operation. The operations may or may not affect, or may ormay not substantially affect, the CO and/or CO₂ concentration ofmethanation reactor feed 212 (methanation reactor inlet CO concentrationor methanation inlet CO₂ concentration), as described above (e.g., theymay cause a change (an increase or a decrease) in the absoluteconcentration, or alternatively the relative amount, of CO and/or CO₂from the inlet of the operation to the outlet of the operation(excluding any flow bypassing the operation) in the ranges as describedabove). Alternatively, or additionally, the operations may or may notaffect, or may or may not substantially affect, the CO/H₂ molar ratioand/or CO₂/H₂ molar ratio of methanation reactor feed 212 (methanationreactor inlet CO/H₂ molar ratio or CO₂/H₂ molar ratio), as describedabove (e.g., they may cause a change (an increase or a decrease) in theabsolute molar ratio, or alternatively the relative molar ratio, ofCO/H₂ and/or CO₂/H₂ from the inlet of the operation to the outlet of theoperation (excluding any flow bypassing the operation) in the ranges asdescribed above).

In the embodiment of FIG. 2, some operations are shown as beingperformed in gasification zone 100, whereas others are shown as beingperformed in methanation zone 200, with the demarcation between thesezones 100, 200 being compressor 205 that increases the process pressurefrom that associated with conditions in gasifier 105 (minus any pressurelosses occurring in equipment between gasifier 105 and compressor 205)to that associated with conditions in methanation reactor 225 (plus anypressure losses occurring in equipment between compressor 205 andmethanation reactor 225). Therefore, it can be appreciated that,according to some embodiments, operations in gasification zone 100 maybe performed at relatively low pressures, for example at an absolutepressure generally from about 0.1 megapascals (MPa) (14.5 psi) to about3 MPa (435 psi), whereas operations in methanation zone 200 may beperformed at relatively high pressures, for example at an absolutepressure from about 2 MPa (290 psi) to about 6.5 MPa (943 psi). However,those skilled in the art, having knowledge of the present disclosure,will recognize that one or more of the operations shown in gasificationzone 100 may alternatively be performed in methanation zone 200 and viceversa. Operations may be performed in different orders, relative to theorder shown, or otherwise may be excluded. In addition, the depictedoperations do not preclude the possibility of one or more additionalsteps, or operations, occurring upstream and/or downstream of a givenillustrated operation, such as reaction (e.g., COS hydrolysis), input orrecovery of heat, removal of water, mixing with an auxiliary feedstream, separation of an intermediate product stream (e.g., for NH₃removal), bypassing one or more operations with a portion of a processstream, and/or recycle of a portion of a process stream back to the sameoperation or an upstream operation.

Representative processes may therefore comprise one or more of thespecific operations illustrated in FIG. 2, depending at least partiallyon the characteristics of carbonaceous feed 10 and conditions ingasifier 105, both of which affect and the composition of gasifiereffluent 103 and consequently the subsequent processing operationsneeded in gasification zone 100 and/or methanation zone 200, forproviding a suitable composition of methanation reactor feed 212. In theparticular embodiment illustrated in FIG. 2, (i) gasifier effluent 103,which is alternatively a tar conversion feed, is processed in tarconversion operation 110 to provide hot tar conversion product 106,having a reduced concentration of tar; (ii) hot tar conversion product106, which is alternatively a gasifier heat recovery feed, is cooled ingasifier heat recovery operation 115 to provide gasifier heat recoveryproduct 109, having a lower temperature, in addition to gasifier heat118, as described above; (iii) gasifier heat recovery product 109, whichis alternatively a gas filtration/scrubbing feed, is processed in gasfiltration/scrubbing operation 120 to provide gas filtration/scrubbingproduct 112, having a reduced solid particle content; (iv) gasfiltration/scrubbing product 112, which is alternatively a supplementalcleaning feed, is processed in supplemental cleaning operation 125 toprovide a supplemental cleaning product, for example as gasificationzone effluent 16, as described above, having a reduced concentration ofcondensable hydrocarbons (e.g., benzene and/or naphthalene); (v)gasification zone effluent 16, which is alternatively a methanation zonefeed (or combined methanation zone feed 20, as described above, ifcombined with at least a portion of electrolysis hydrogen 18), iscompressed in compressor 205 to provide compressed product 203, having ahigher pressure; (vi) compressed product 203, which is alternatively asour shift feed, is processed in sour shift operation 210 to providesour shift product 206, having a higher concentration of H₂; (vii) sourshift product 206, which is alternatively a sulfur removal feed, isprocessed in sulfur removal operation 215 to provide sulfur removalproduct 209, having a reduced concentration of sulfur compounds (e.g.,H₂S and/or COS); and (viii) sulfur removal product 209, which isalternatively an acid gas removal feed, is produced in acid gas removaloperation 220 to provide an acid gas removal product, for example asmethanation reactor feed 212, as described above, and having a lowerconcentration of CO₂. As described above, methanation reactor feed 212is processed in methanation reactor 225 to react CO and CO₂ with H₂ andthereby provide hot methanation product 216, having a higherconcentration of methane. Downstream of methanation reactor 225, hotmethanation product 216, which is alternatively a methanation reactorheat recovery feed, is cooled in methanation reactor heat recoveryoperation 230 to provide methanation reactor effluent 25, having a lowertemperature, in addition to methanation reactor heat 218, as describedabove. Hot methanation product 216 and/or methanation reactor effluent25 may be subjected to drying, and water that is removed in a dryingstep may be recovered and used, as described above, to provide all or aportion of water 30 for electrolysis.

As illustrated in FIG. 2, electrolysis hydrogen 18, as an exemplarysource of makeup hydrogen, may be combined with gasification zoneeffluent 16, such that only a single compressor 205 may be required fordownstream methanation. Alternatively, and as described above,electrolysis hydrogen 18 may be combined with any of compressed product203, sour shift product 206, sulfur removal product 209, and/ormethanation reactor feed 212, for example using a supplemental source ofcompression (not shown). Combining electrolysis hydrogen 18 with sourshift product 206, for example, may improve the conversion to hydrogenin sour shift operation 210, in view of equilibrium limitations of theWGS reaction. Combining electrolysis hydrogen 18 with either sulfurremoval product 209, and/or methanation reactor feed 212 may reduce theoverall flows through sulfur removal operation 215 and/or acid gasremoval operation 220 and thereby result in lower capital and operatingcosts due to correspondingly reduced equipment sizes for theseoperations.

Gasification zone 100 may include one or more gasifiers (e.g., operatingin series or parallel), operating under gasifier conditions (e.g.,conditions present in a gasification reactor) that include a temperatureof generally from about 500° C. (932° F.) to about 1000° C. (1832° F.),and typically from about 750° C. (1382° F.) to about 950° C. (1742° F.).Often, a gasifier is operated with a gasification reactor having afluidized bed of particles of the carbonaceous feed, with theoxygen-containing gasifier feed, and optionally H₂O— and/orCO₂-containing feeds, being fed upwardly through the particle bed. Othergasification reactor configurations include counter-current fixed bed(“up draft”), co-current fixed bed (“down draft”), and entrained flowplasma. Gasifier conditions may also include atmospheric pressure orelevated pressure, for example an absolute pressure generally from about0.1 megapascals (MPa) (14.5 psi) to about 10 MPa (1450 psi), andtypically from about 1 MPa (145 psi) to about 3 MPa (435 psi). Variouscatalytic materials may be used in a gasification reactor, includingsolid particles of dolomite, supported nickel, alkali metals, and alkalimetal compounds such as alkali metal carbonates, bicarbonates, andhydroxides. Different catalysts, having differing activities for one ormore desired functions in gasification, such as tar reduction, enhancedH₂ yield, and/or reduced CO₂ yield, may be used. Limestone may be addedto a gasification reactor, for example, to promote tar reduction bycracking. The gasifier effluent, which may also be referred to assyngas, generally contains CO, CO₂, H₂, water, methane, sulfur compounds(e.g., H₂S and/or COS), nitrogen compounds (e.g., NH₃), tar, and solidparticulates. According to representative embodiments, the gasifiereffluent H₂ concentration is generally from about 10 mol-% to about 55mol-%, and typically from about 20 mol-% to about 45 mol-%. The CO andCO₂ concentrations are each generally from about 5 mol-% to about 50mol-%, and typically from about 15 mol-% to about 35 mol-%. The methaneconcentration is generally from about 1 mol-% to about 15 mol-%, andtypically from about 2 mol-% to about 10 mol-%. The gasifier effluentmay comprise mainly (e.g., in a combined concentration of at least about75 mol-%, at least about 80 mol-%, or at least about 85 mol-%) thecomponents CO, CO₂, water, and H₂.

Tar conversion operation 110 may be used to reduce the concentration oftar (organic compounds having a boiling point temperature greater thanthat of benzene) in the gasifier effluent, produced by the gasification.Compounds removed by tar conversion, or tar reforming, include C₆ ⁺hydrocarbons (e.g., toluene and/or naphthalene) and oxygenates such asphenol. In the absence of tar conversion, the amount of tar produced bybiomass gasification can be sufficient, over time, to cause seriousproblems in downstream equipment such as corrosion and/or plugging. Thetar conversion operation may include a separate reactor, containing abed of catalyst comprising solid or supported Ni, solid or supported Fe,and/or dolomite, for example in the form of a secondary fluidized beddownstream of a gasifier. Other catalysts for tar conversion includeolivine, limestone, zeolites, and even metal-containing char producedfrom the gasification. Tar may alternatively be converted bynon-catalytic partial oxidation. According to one embodiment, tar may beconverted by both cracking and steam reforming to beneficially increasethe yield of H₂ and CO₂, initially obtained from gasification, oftenwhile maintaining the yield of methane, initially obtained. Tarconversion reactions may be performed at temperatures higher than thoseused in the gasifier, such as above 1000° C. (e.g., from about 1000° C.(1832° F.) to about 1250° C. (2282° F.)).

Gas filtration/Scrubbing operation 120 may be used to remove solidparticles (particulates) in a gasifier effluent. In the case of biomassgasification, the solid particles formed include char, tar, soot, andash, any of which can generally contain alkali metals such as sodium.Corrosive and/or harmful species such as chlorides, arsenic, and/ormercury may also be contained in such particulates. A high temperaturefiltration, for example using bundles of metal or ceramic filters, maygenerally be sufficient to reduce the content of particulates in thegasifier effluent to less than 1 wt-ppm, and possibly less than 0.1wt-ppm. In some embodiments, gas filtration/scrubbing operation 120 maybe performed upstream of (prior to) tar conversion operation 110 toallow the latter to operate more effectively.

Supplemental cleaning operation 125 may be used to further reduce thetar and overall hydrocarbon content of a gasifier effluent, for exampleby contact with a solid “polishing” material such as a carbon bed. Thiscan provide for more thorough removal of benzene, naphthalene, toluene,phenols, and other condensable species that could otherwise bedetrimental to the operation of compressor 205 and/or could depositonto, and thereby cause deactivation of, catalyst used in themethanation reactor (e.g., by blockage of catalytically active Ni sitesof the catalyst).

Sour shift operation 210 may be used to perform a WGS reaction, in thepresence of sulfur compounds, and thereby increase the concentration ofH₂ (or the H₂:CO molar ratio) relative to that initially in a gasifiereffluent, as obtained from a gasifier. This operation may include one ormore WGS reactors (e.g., operating in series or parallel) having asuitable catalyst that is resistant to deactivation in the presence ofH₂S and/or COS, such as a cobalt-molybdenum catalyst. Other catalystsfor this purpose include those based on copper-containing and/orzinc-containing catalysts, such as Cu—Zn—Al; chromium-containingcatalysts; iron oxides; zinc ferrite; magnetite; chromium oxides; andany combination thereof (e.g., Fe₂O₃—Cr₂O₃ catalysts). Conditions forthe catalytic WGS reaction include a temperature from about 150° C.(302° F.) to about 400° C. (752° C.).

Sulfur removal operation 215 may be used to reduce the concentration ofsulfur compounds, including H₂S, COS, and/or SO₂, in a gasifiereffluent. These compounds, which result from the presence of tracequantities of sulfur in carbonaceous feeds, including biomass, may bedetrimental to (e.g., poison) catalyst used in a methanation reactor,for example due to the formation of nickel sulfide at catalyticallyactive Ni sites of the catalyst. The sulfur removal operation mayinclude contacting the gasifier effluent at any stage of the process(e.g., downstream of a sour shift operation as shown in FIG. 2) with aguard bed suitable to obtain a product that is essentially free ofsulfur, for example having less than 1 wt-ppm, such as less than 0.1wt-ppm, of total sulfur. Suitable guard bed materials include those usedin water treatment, such as iron-containing sorbent (iron “sponge”)materials and/or zinc oxide.

Acid gas removal operation 220 may be used to reduce the concentrationof CO₂ and/or other acid gases (e.g., H₂S) relative to the respectiveconcentrations initially in a gasifier effluent, as obtained from agasifier, and/or relative to the respective concentrations obtained in asour shift product. The acid gas removal operation may utilize one ormore stages of contacting with a physical solvent such as Selexol®(dimethyl ethers of polyethylene glycol), Rectisol® (cold methanol), ora combination thereof. One or more amine solvents such asmonoethanolamine, diethanolamine, methyldiethanolamine,diisopropylamine, or diglycolamine, or otherwise methanol, potassiumcarbonate, a solution of sodium salts of amino acids, etc. can also beused to remove at least a portion of an acid gas. An acid gas removalproduct, upstream of a methanation reactor, may have a CO₂ concentrationgenerally from about 2 mol-% to about 40 mol-%, and typically from about5 mol-% to about 20 mol-%, and may have a total sulfur concentration ofless than about 0.1 mol-ppm.

Methanation zone 200 may include one or more methanation reactors (e.g.,operating in series or parallel) to react CO and/or CO₂ with hydrogenand thereby provide a hot methanation product, and/or ultimately amethanation reactor effluent, having a significantly higherconcentration of methane, relative to that initially in a gasifiereffluent, as obtained from a gasifier. Catalysts suitable for use in amethanation reactor include supported metals such as ruthenium and/orother noble metals, as well as molybdenum and tungsten. Generally,however, supported nickel catalysts are most cost effective. Often, amethanation reactor is operated using a fixed bed of the catalyst. Otherconditions suitable for a methanation reactor include a temperature ofgenerally from about 250° C. (482° F.) to about 600° C. (1112° F.), andtypically from about 350° C. (662° F.) to about 500° C. (932° F.).Methanation conditions may also include an absolute pressure generallyfrom about 1.4 (MPa) (203 psi) to about 6.9 MPa (1000 psi), andtypically from about 2.8 MPa (406 psi) to about 5.5 MPa (798 psi). Theconcentration of methane in a methanation reactor effluent (e.g., an RNGproduct), or a product recovered from this effluent, may be generally atleast about 60 mol-% (e.g., from about 60 mol-% to about 99.9 mol-%),and typically at least about 80 mol-% (e.g., from about 80 mol-% toabout 99 mol-%). Such effluents and/or products may have otherproperties of “pipeline quality” methane, as described above.

Overall, aspects of the invention relate to improvements in theutilization of hydrogen, for example electrolysis hydrogen as a makeupsource that is supplied in conjunction with a stoichiometric amount ofelectrolysis oxygen to integrated gasification and methanationprocesses, for the production of methane. Those skilled in the art,having knowledge of the present disclosure, will recognize that variouschanges can be made to these processes in attaining these and otheradvantages, without departing from the scope of the present disclosure.As such, it should be understood that the features of the disclosure aresusceptible to modifications and/or substitutions, and the specificembodiments illustrated and described herein are for illustrativepurposes only, and not limiting of the invention as set forth in theappended claims.

The invention claimed is:
 1. A process for producing methane, theprocess comprising: in a gasifier, contacting a carbonaceous feed withan oxygen-containing gasifier feed to provide a gasifier effluentcomprising CO, CO₂ and H₂; in a methanation reactor, reactingelectrolysis hydrogen, obtained from an electrolyzer, with at least aportion of the CO and/or CO₂ in the gasifier effluent to form methane,and adjusting an operation of the process, affecting a methanationreactor inlet CO concentration or a methanation reactor inlet CO₂concentration, in response to a makeup quantity of the electrolysishydrogen, wherein the operation is upstream of the methanation reactorand not a recycle of a portion of a process stream back to an upstreamoperation.
 2. The process of claim 1, further comprising obtaining amethane product as, or recovering the methane product from, themethanation reactor effluent.
 3. The process of claim 1, wherein theoperation of the process, affecting the methanation reactor inlet COconcentration or the methanation reactor inlet CO₂ concentration, (i)consumes or produces CO or CO₂ in the process, or (ii) adds or removesCO or CO₂ in the process.
 4. The process of claim 3, wherein theoperation of the process, affecting the methanation reactor inlet COconcentration or the methanation reactor inlet CO₂ concentration, is asour shift operation that consumes CO and produces CO₂ in the process,or an acid gas removal operation that removes CO and CO₂ in the process.5. The process of claim 1, wherein the operation is adjusted bybypassing at least a portion of a feed to the operation.
 6. The processof claim 1, wherein the operation is adjusted to obtain a methanationreactor inlet ratio of [H₂]/(3.[CO]+4.[CO₂]) from about 0.7 to about1.5.
 7. The process of claim 1, wherein the carbonaceous feed comprisescoal or biomass.
 8. The process of claim 1, wherein theoxygen-containing gasifier feed comprises electrolysis oxygen, obtainedfrom the electrolyzer.
 9. The process of claim 8, wherein theoxygen-containing gasifier feed further comprises ASU oxygen obtainedfrom an air separation unit.
 10. The process of claim 9, wherein theelectrolysis oxygen and the ASU oxygen provide a makeup quantity ofoxygen that is determined at least partly on the basis of a makeupquantity of the carbonaceous feed.
 11. The process of claim 1, whereinthe oxygen-containing gasifier feed further comprises H₂O and/or CO₂.12. The process of claim 1, further comprising one or more additionaloperations, downstream of the gasifier and upstream of the methanationreactor, the one or more additional operations performing respective oneor more processing steps on the gasifier effluent, the processing stepsselected from the group consisting of converting tar, recovering heat,filtering/scrubbing, supplemental cleaning, and removing sulfur.
 13. Anintegrated gasification, methanation, and electrolysis process forproducing renewable natural gas, the process comprising: in a gasifierof a gasification zone, contacting biomass with an oxygen-containinggasifier feed to provide a gasification zone effluent comprising CO, CO₂and H₂, wherein the oxygen-containing gasifier feed compriseselectrolysis oxygen obtained from electrolysis of water in anelectrolyzer; in a methanation reactor of a methanation zone, convertingat least a portion of the CO and/or the CO₂ in the gasification zoneeffluent to methane; and obtaining the renewable natural gas as, orrecovering the renewable natural gas from, a methanation zone effluent,wherein the gasification zone or the methanation zone comprises at leastone operation affecting CO concentration or CO₂ concentration, theprocess further comprising adjusting the operation in response to amakeup quantity of electrolysis hydrogen obtained from the electrolyzer,and wherein the operation is upstream of the methanation reactor and nota recycle of a portion of a process stream back to an upstreamoperation.
 14. The process of claim 13, wherein the gasification zonecomprises, in addition to the gasifier, one or more operations selectedfrom the group consisting of a tar conversion operation, a gasifier heatrecovery operation, a gas filtration/scrubbing operation, and asupplemental cleaning operation.
 15. The process of claim 13, whereinthe methanation zone comprises, in addition to the methanation reactor,one or more operations selected from the group consisting of a sourshift operation, a sulfur removal operation, an acid gas removaloperation, and a methanation reactor heat recovery operation.
 16. Theprocess of claim 15, wherein the at least one operation affecting COconcentration or CO₂ concentration is the sour shift operation or theacid gas removal operation.
 17. The process of claim 13, furthercomprising combining at least a portion of the gasification zoneeffluent with electrolysis hydrogen obtained from the electrolyzer, forreaction in the methanation zone to provide at least a portion of themethane in the methanation zone effluent.
 18. The process of claim 17,wherein the electrolysis hydrogen increases the yield of methane basedon carbon present in the biomass, by at least about 10% relative to abaseline process performed without addition of the electrolysishydrogen.
 19. The process of claim 13, wherein the renewable natural gasobtained as, or recovered from, the methanation zone effluent, ispipeline quality natural gas.
 20. A process for producing methane, theprocess comprising: gasifying a carbonaceous feed to provide a gasifiereffluent comprising CO, CO₂ and H₂; introducing a makeup quantity ofhydrogen to the methanation reactor or upstream of the methanationreactor, to react at least a portion of the makeup hydrogen with atleast a portion of the CO and/or CO₂ in the gasifier effluent to formmethane, and adjusting an operation of the process in response to themakeup quantity of hydrogen, wherein the operation is upstream of themethanation reactor, and further wherein the operation is not a recycleof a portion of a process stream back to an upstream operation.
 21. Theprocess of claim 20, wherein the process excludes at least oneprocessing step on the gasifier effluent of sour shifting or removingacid gas.
 22. A process for gasification of a carbonaceous feed, theprocess comprising: in a gasifier, contacting the carbonaceous feed withan oxygen-containing gasifier feed, performing a series of operationsdownstream of the gasifier to provide a gasifier effluent comprising CO,CO₂ and H₂, wherein at least one of said series of operations causes achange in a concentration of CO and/or CO₂ from an inlet of said atleast one operation to an outlet of said at least one operation, orotherwise causes a change in a relative amount of CO and/or CO₂ from theinlet of said at least one operation to the outlet of said at least oneoperation, the process further comprising: adjusting said at least oneof said series of operations, in response to a makeup quantity ofelectrolysis hydrogen, said makeup quantity being provided by anelectrolyzer that further provides a makeup quantity of oxygen for saidoxygen-containing gasifier feed, wherein said adjusting is performed tomaintain a CO/H₂ molar ratio of the gasifier effluent, followingcombining of said makeup quantity of electrolysis hydrogen with aproduct of any of said series of operations.
 23. The process of claim22, wherein said makeup quantity of oxygen is determined based on both amakeup quantity of the carbonaceous feed and a characteristic of thecarbonaceous feed.
 24. The process of claim 23, wherein thecharacteristic is a carbon content of the carbonaceous feed.
 25. Theprocess of claim 22, wherein said at least one operation of the processis a sour shift operation that consumes CO and produces CO₂ in theprocess, or an acid gas removal operation that removes CO and CO₂ in theprocess.
 26. The process of claim 22, wherein said at least oneoperation is adjusted by bypassing at least a portion of a feed to theoperation.
 27. The process of claim 22, wherein said series ofoperations perform one or more of converting tar, recovering heat,filtering/scrubbing, supplemental cleaning, and removing sulfur, toprovide said gasifier effluent.